Solid-porous polymeric elastomer articles combine such valuable properties as limited stiffness and high elasticity, high abrasion resistance, good oxygen ageing, high rupture resistance, and great hardness.
That notwithstanding, extensive application of such articles is limited by the complexity of the present-day methods for producing a solid portion and a porous portion in one article, which involves the use of different polymeric elastomer compounds and different ways of joining these portions together, of which glueing is an example. Another limiting factor is the impossibility of obtaining solid-porous polymeric rubber-like articles of a required configuration.
There is known a method for producing variable-density articles from polyurethane. According to this method, one type of raw polymer elastomeric compound containing a blowing agent is used to produce an article with at least two portions having different densities.
Raw polymer elastomeric compound containing a blowing agent is put into a press mold. One portion of said press mold (Portion 1) is provided with means to limit the foaming of the polyurethane. The other portion of the press mold (Portion 2) has no such means. The raw elastomer compound is heated at temperatures ranging from 43.3.degree. to 204.degree. C. As it is being heated, the raw compound foams, producing an article of a partially polymerized substance, said article having two portions of a different height, but of an equal specific weight. The part of the article that is found in Portion 2 of the press mold is then compressed so that its height becomes equal to that of the part in Portion 1 of the press mold, the compression being maintained over a period of time of up to 10 min, after which the pressure is brought back to normal.
The foregoing method makes it possible to manufacture molded polyurethane articles having two portions with different densities. The borderline between said portions is practically parallel to the direction of the compression.
One of disadvantages inherent in the above method is the fact that the foaming of the molded blank, which takes place at the early stage of the process, only makes it possible to obtain a porous article with variable density. The shape of the portions of this porous article which have different densities is determined by that of Portion 1 of the press mold and the direction of the compression. In addition, the method necessitates the use of telescopic press molds of complicated shapes, which rules out the possibility of carrying out a continuous process.
There is known a number of processes for manufacturing solid-porous articles, for example, a method for manufacturing three-layer shoe soles. The latter method provides for manufacturing three-layer shoe soles whose upper and lower layers are solid, whereas the intermediate layer is porous. Such a sole is made from a molded blank consisting of three layers of a raw polymeric rubber-like compound, the intermediate layer differing from the upper and lower ones by the presence of a blowing agent therein. During the initial stage of the process, the molded blank is partially vulcanized in a press mold at a pressure of 20 to 30 kg/cm.sup.2 and a temperature of about 160.degree. C., which temperature is either equal to or higher than the decomposition temperature of the blowing agent. During the second stage of the process, the pressure is (released), the compound containing the blowing agent foams, and the vulcanization of the blank is completed.
The article thus obtained has solid and porous portions.
Apart from inconveniences involved in the preparation of two different polymeric compounds or compositions, the manufacturing of three-layer shoe soles necessitates a number of laborious operations, such as coating the intermediate layer on both sides. The process also calls for expensive press molds and other equipment. In addition, defects often occur in joining the layers together, which in most cases is due to the formation of air bubbles between layers. Finally, the use of the press molding operation makes it absolutely impossible to carry out a continuous manufacturing process.